This invention relates to a control system which controls a multi-ratio vehicle transmission.
It is known to automatically control the gear ratio of a vehicle transmission as a function of various sensed operating parameters. For example, it is known to automatically control a transmission to maintain the engine speed or transmission speed within narrow ranges. It is also known to provide a transmission control system with selectable manual and automatic control modes. However, such transmission control systems have not provided the capability of automatically switching from an automatic control mode to a purely manual control mode in response to various conditions, such as in response to a certain number of unsuccessful attempts to shift automatically, or such as in response to a single manual gear selecting operation. For example, the transmission control system disclosed in U.S. Pat. No. 3,628,642 has both manual and automatic control modes. However, with this control system, the manual control mode is limited by the electronic control unit with respect to an engine overspeed condition. Similarly, with the transmission control system described in U.S. Pat. No. 4,208,925, forced downshifting is delayed by the controller until vehicle ground speed is below a downshift speed point. Therefore, these control systems interfere with an operator attempting an emergency or forced downshift for the purpose of braking with the engine, and thus the manual control capability provided thereby is less than total.